1. Field of the Invention
The invention relates generally to high voltage switching power supplies and in particular to those high voltage switching power supplies for use with beam penetration type color cathode-ray tubes. Such cathode-ray tubes generally have two or more layers of phosphors forming a phosphorescent screen, the color emitted from the screen being dependent upon the voltage aplied to the screen electrode. Once such cathode-ray tube and system employing the same is shown in U.S. Pat. No. 3,840,773 issued Oct. 8, 1974 to Harold M. Hart and assigned to the present assignee.
One of the most significant problems encountered with use of such tubes is the rate at which the voltage applied to the screen anode can be changed. The screen anode forms a relatively large capacitance with the surrounding grounded structure making it necessary to charge and discharge the capacitor as the voltage applied to the screen anode is changed. Use of beam penetration type cathode-ray tubes has heretobefore been restricted due to the large power requirements and attendant high cost of prior high voltage switching power supplies.
2. Description of the Prior Art
The earliest approach for providing a variable high voltage source for beam penetration cathode-ray tube was the use of a standard variable voltage high voltage power supply which tracked an input voltage indicative of the voltage applied to the final accelerating anode of the beam penetration cathode-ray tube. Switching times between colors were excessively long using this type of power supply. Often, switching times were so slow that it was not possible to switch between colors more than one time during each display refresh cycle. All data and patterns to be displayed in one particular color had to be written upon the screen of the cathode-ray tube before the color changed and be other data and patterns of other colors could be displayed. Such systems could not readily be used for the display of, for example, radar data where signals are continuously received from a radar processor during a refresh cycle and the color of individual sections of data to be displayed is not a priori known.
Later, systems were developed having a low impedance power supply selectively coupled to two accelerating electrodes of the beam penetration cathode-ray tube through silicon controlled rectifier switches. One such system is described in U.S. Pat. No. 3,492,416 issued Jan. 20, 1970 to R. L. Weber. These systems suffered from a number of disadvantages. Firstly, the voltage applied to two separate accelerating electrodes had to be varied in a predetermined nonlinear manner. Switching between colors was not achieved particularly rapidly as energy was transferred to the accelerating electrodes only as a sequence of pulses from a resonant circuit formed with a coupling transformer. Moreover, as the silicon controlled rectifiers used therein had to be turned off by current reversal achieved through a resonant circuit, the amount of energy transferred to the anodes was limited to discreet amounts determined by the energy content of the various pulses. Hence, it was somewhat difficult to control the precise voltage applied to the anodes. Moreover, there was no provision made in these systems for automatically compensating the gain of the deflection amplifiers for changes in the deflection factor the tube caused by varying the voltages applied to the accelerating electrodes.
Later, systems were developed in which energy stored in inductors was transferred to the accelerating electrode of a beam penetration cathode-ray tube. Such a system is described in U.S. Pat. No. 3,780,339 issued Dec. 18, 1973 to Alfred J. Mayle. That system required precisely controlled equal and opposite voltage sources applied to the accelerating anode and to the cathode of the cathode-ray tube shown therein. Changing colors was achieved by changing the polarity of the voltage applied to the accelerating electrode relative to ground potential. The cathode of the tube had to be operated at a high voltage making it necessary that the video signals and other operating voltages applied to the tube be biased with the same high voltage. Such operation necessarily complicated the construction of such circuits and made them more expensive than they would otherwise be except for the presence of the high biasing voltage. Also, such a system is only capable of operating between two different voltages and hence can only reproduce two different colors.